1. Field of the Invention
The present invention relates to a method and an apparatus for producing a silicon carbide (SiC) crystal used for a material of a semiconductor, a light emitting diode and the like.
2. Description of Related Art
SiC crystals are typically used in semiconductors, light emitting diodes and other crystal accepting devices and applications. While current crystal producing methods have generally proven to be satisfactory for their applications, each is associated with its share of limitations. One major limitation with many current crystal producing methods and apparatuses is their inability to produce a crystal in a controlled form to achieve a precise shape and size of crystal.
The problem of producing application SiC crystals in a conventional method has been addressed by the prior art and is disclosed in JP-A-8-295595 as the following method. That is, a Sic material is disposed at a lower portion of a container such as a graphite crucible while a SiC seed crystal is disposed at an upper portion thereof. The temperature of the SiC seed crystal is made lower than a temperature of the SiC material. By sublimation, a SiC gas emits from the Sic material and is supplied onto the SiC seed crystal to make the SiC crystal grow. The SiC crystal grows downward so that its diameter is enlarged in a divergent shape. Since the SiC crystal finally becomes a bulk shape different from a shape to be used, the bulk shape is required to be suitably changed to a desired, usable shape, such as a wafer. However, since a SiC crystal is very hard, changing its shape is difficult. This inability to produce a crystal in a precise shape and size has resulted in additional crystal processing to achieve the precise shape and size crystal for the given application.
Another major limitation with current crystal producing methods and apparatuses is their inability to produce a crystal free of crack defects which generally originate in a crack core. In general, prior art methods of producing a SiC crystal results in crystal growth speeds that are different between a center portion and a peripheral portion of a crystal growing surface. The center portion of a SiC crystal ingot, having grown, may either protrude or recess relative to its peripheral surface. Therefore, crystal defects, especially crack defects, are generated in the growing SiC crystal.
Therefore, a generation mechanism of a crack defect is considered. When a crystal growing surface of the SiC crystal to grow is irregular, its cross-sectional shape becomes different from a circle. Therefore, a circle is supposed as the cross-sectional shape of the SiC crystal as in FIG. 13A, and the generation mechanism of the crack defects will be described in this circular model.
Since atoms are liable to be arranged on a plane  less than 1{overscore (1)}00 greater than , the SiC crystal grows slower in a direction  less than 11{overscore (2)}0 greater than  than in a direction  less than 1{overscore (1)}00 greater than  as shown in FIG. 13B. Therefore, when it is supposed that the SiC crystal grows in a large free space, as the circular SiC crystal shown in FIG. 13A is enlarged in its radial direction, it becomes a hexagon shape which protrudes in the direction  less than 11{overscore (2)}0 greater than .
As shown in FIG. 13B, atoms (indicated by arrows S) moving on the plane  less than 1{overscore (1)}00 greater than  collide with each other due to defects and some lattice mismatching so that a crack core is generated at their collision position (indicated by an arrow T). As the SiC crystal grows, crack defects are generated to extend in the directions  less than 1{overscore (1)}00 greater than  and  less than 11{overscore (2)}0 greater than , as indicated by arrows U shown in FIG. 13C, from the collision core.
What is needed then is a crystal producing method and apparatus that does not suffer from the above limitations. This in turn, will provide a method and apparatus that eliminates the problem of producing SiC crystals that are of a size and shape that require extensive additional processing after forming to produce them in the shape of a wafer, and provide a SiC crystal producing method and apparatus that is capable of producing a SiC crystal free of crack defects, even those originating at a crack core.
In accordance with the teachings of the present invention, a method and apparatus for producing silicon carbide (Sic) crystal is disclosed. The present invention has been made in view of the above problems. Thus, a first object of the present invention is to produce a SiC crystal in a precise size and shape similar to a wafer, so the Sic crystal can be used in a manner similar to that in which a Sic wafer is used. A second object of the present invention is to restrict and control the crack cores, which prevent the generation of crack defects.
In order to achieve the objects of the present invention, there is provided a wall member disposed around the SiC crystal substrate so as to cover a peripheral portion of the SiC crystal substrate. This acts to control the growth of the SiC crystal toward the wall member. Additionally, by setting the temperature of the wall member higher than the sublimation temperature of the SiC when the Sic crystal is grown, SiC crystal growth is controlled. In this manner, a peripheral portion of the SiC crystal is covered by the wall member, so that the SiC crystal growth can be controlled in a radial direction with the wall member having a high temperature which controls the shape of the SiC crystal. Producing a Sic crystal in a wafer shape is desirable to eliminate any post-production forming or machining.
In one example, the wall member is constructed in a substantially circular tube shape whose inner diameter is larger than a diameter of the SiC crystal substrate, and the SiC crystal substrate is disposed within a hollow of the wall member. Therefore, the SiC crystal can be produced in a substantially cylindrical shape. When the wall member is formed in a substantially circular tube shape but with a cross-section having an arc portion corresponding to an orientation flat, the SiC crystal having an orientation flat (flat portion) can be produced. When the wall is formed in a substantially hexagonal tube shape and the silicon carbide crystal substrate is disposed within a hollow of the wall member, the SiC crystal can be produced in a substantially hexagonal pole shape.
When alignment is made between a shape of the wall member and any one specific direction of the SiC crystal substrate, the SiC crystal can be produced in a shape according to the alignment. When the wall member is formed in a cone shape so that the SiC crystal is enlarged in a radial direction as it grows, its diameter can be controlled. The wall member is formed in a cone shape so that its cross-section is substantially circular around the SiC crystal substrate and is changed into a hexagonal shape in a crystal growing direction of the SiC crystal. Therefore, a cross-sectional shape of the SiC crystal can be changed as it grows. For instance, the wall member can be formed in a cone shape so as to be inclined at 5 to 80 degrees with respect to a direction perpendicular to a surface of the SiC crystal substrate. The wall member is formed so that a clearance between the wall member and the SiC crystal substrate is set at 5 to 30 percent of a radius of the SiC crystal substrate. Therefore, SiC crystal growth is possible in the radial direction utilizing the heat of the wall member which facilitates the growth of the SiC crystal.
Further, the method of producing a SiC crystal according to the present invention can be performed using an apparatus. A SiC seed crystal is disposed in a container and a SiC crystal is grown on the SiC crystal substrate by supplying sublimation gas from a SiC material onto the SiC crystal substrate. Further, a wall member having a hexagonal hollow is disposed so as to cover a peripheral portion of the SiC crystal substrate, and the SiC crystal grows along the inner wall surfaces defining the hexagonal hollow. In this manner, since the SiC crystal grows along the inner wall surfaces defining the hollow, an ingot of the SiC crystal can be formed in an ideal hexagonal pole shape constructed by flat crystal growing surfaces, thereby preventing the generation of crack defects growing from crack cores.
The SiC crystal substrate is disposed so that its planes coincide with the inner wall surfaces of the wall member thereby defining a hexagonal pole hollow. Therefore, generation of cracks due to atomic collisions within the crystal growing surface of the SiC crystal can be prevented. Accordingly, even when a radial dimension of the SiC crystal substrate is smaller than an inner radial dimension between the inner wall surfaces defining the hollow of the wall member, generation of cracks due to atomic collision within the crystal growing surface of the SiC crystal can be prevented.
The SiC crystal substrate has the planes of a particular form to create a substantially hexagonal pole shape. Accordingly, a shape of the SiC substrate conforms to a shape of the SiC crystal, thereby restricting crack generation during an initial crystal growth. A dimension of the wall member is made larger than a dimension of the SiC crystal during its growth, so that the SiC crystal grows along the inner wall surfaces of the wall member, thereby making its growing surface flat. Similarly, a dimension of the wall member is made smaller than a dimension of the SiC crystal, having grown in the growing direction of the SiC crystal, and the SiC crystal is grown so as to have planes forming a substantially hexagonal pole shape after passing along the wall member.
Although the SiC crystal having grown along the wall member in an ideal shape, has sufficient free space, the SiC crystal continues to grow while the substantially hexagonal pole shape, constructed by the planes of a particular form, is maintained. Accordingly, a SiC crystal ingot can be formed in an ideal hexagonal pole shape constructed by flat crystal growing surfaces, thereby preventing the crack cores from generating the crack defects.
Furthermore, the SiC crystal grows along the inner wall member surface after the SiC crystal grows to reach the wall member. This occurs when an inner radial dimension between the inner wall surfaces defining the hollow of the wall member is made smaller than a radial dimension of the SiC crystal substrate, and the wall member is disposed forward of the SiC crystal substrate in the growing direction of the SiC crystal. According to this manner as well, the SiC crystal, having grown in an ideal shape using the wall member, grows while the substantially hexagonal pole shape constructed by the planes of a form is maintained, thereby restricting the crack cores from generating the crack defects.
The container has a hexagonal pedestal, and the SiC crystal substrate is disposed on the hexagonal pedestal so as to cover the entire pedestal. Since the pedestal is made hexagonal, temperature distribution of the SiC crystal substrate can be uniformly maintained. Each side of the hexagonal SiC crystal substrate is made larger than each side of the hexagonal pedestal. Accordingly, since the temperature of the SiC crystal substrate outside the pedestal becomes high, the SiC crystal growing thereon is sublimated by thermal etching, thereby eliminating crystal defects thereon, and at the crack cores. Since the inner surfaces of the wall member are inclined with respect to the growing direction of the SiC crystal, SiC crystal growth can be restricted or controlled so as to be enlarged in the radial direction. The wall member is formed so that an inner radial dimension between the inner wall surfaces thereof is enlarged in the growing direction of the SiC crystal. Accordingly, when the wall member is constructed at an angle so that the SiC crystal is restricted from being enlarged in the radial direction, crystal defects can be prevented from being generated at a peripheral portion of the SiC crystal.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description, the appended claims and the accompanying drawings.